Conceptual Design Review Stray Light Rob Hubbard Systems Engineering

Definition of Terms Stray light is unwanted light Most problematic when observing faint objects near the sun Possible causes include –Scatter from optical or mechanical surfaces –Ghost reflections –Edge scatter or “glints” –Diffraction around edges

The Science Requirement …The total instrumental scattered light (dust plus mirror roughness) shall be 25 millionths or less at 1000 nm and at 1.1 radii. Values larger than these levels require longer integration times to achieve the desired signal to noise levels.

Relevant Studies Performed Some Stray-Light Reduction Design Considerations for ATST (Andrew Buffington and Bernard V. Jackson, UCSD) M1 Microroughness and Dust Contamination (Rob Hubbard, ATST Systems Engineering) Further Stray-Light Reduction Design Considerations for ATST (Andrew Buffington, UCSD) Advanced Technology Solar Telescope (ATST) Stray and Scattered Light Analysis (Scott Ellis, Richard N. Pfisterer, Photon Engineering, LLC)

“Sunlight reflected from the heat shield/coronagraph occulter does not need to be absorbed nearby, and can be safely dumped into the interior of the building; and…” “Except maybe close to the M1 mirror mount, the building interior can be typical black or even gray paint, without generating significant stray light in the FOV.” “Specifying, manufacturing, testing and certifying M1 could prove a significant challenge for ATST.” “ATST’s success as a coronagraph probably requires aggressive contamination control, even if a low-dust site is found…” Some Stray-Light-Reduction Design Considerations for ATST (July 2002) Buffington and Jackson Conclusions:

Extending the Analysis Can we make additional assumptions that will allow us to better quantify the scattering due to M1 microroughness? Can we refine the dust contamination predictions so that they can be compared to scatter due to M1 microroughness? How frequently will the ATST primary mirror need to be cleaned to maintain acceptable coronagraphic performance?

ATST Technical Note No. 0013

Define a set of parallel rays representing a point source at the position of the sun’s center. Introduce these rays onto a “scatter” surface just in front of the primary mirror (M1). Scatter the parent rays into a half-degree cone centered on the specular direction. Normalize flux so disk center = 1. Add a scatter function to M1 that represents a clean, polished surface, or a surface contaminated by dust. Define a set of parallel rays representing a point source at the position of the sun’s center. Introduce these rays onto a “scatter” surface just in front of the primary mirror (M1). Scatter the parent rays into a half-degree cone centered on the specular direction. Normalize flux so disk center = 1. Add a scatter function to M1 that represents a clean, polished surface, or a surface contaminated by dust. The ASAP Model

The Source Model at 1.1 R sun

Scatter Towards… 1 out of 10,000 delivered 10,000 out of 10,000 delivered

Sample Positions

Typical scatter versus angle for a clean, polished glass surface The Scatter Model

…In direction-cosine space Plotting log 10 | sin  – sin  0 | versus log 10 BSDF

Figure courtesy of Gary Peterson, Breault Research Organization; measurement by James Harvey. Harvey Model  = 0.57º

Even and small angles? arcmin

Power Spectral Density Church, Eugene L.,” Fractal Surface Finish,” (Applied Optics 27, No. 8, 15 April 1998.) ~40 arcsec (from grating equation)

Profile of a Star

The Profilometer and Roughness !

Microroughness and Harvey The single RMS roughness parameter (  ) contains insufficient information to completely characterize the BSDF of the polished surface, even assuming a power- law relationship.

Slope Ranges Angle (Degrees)

= 1.0 Micrometer The 20 Ångstrom Finish

= 1.0 Micrometer The Likely Finish

R/R sun S = –1.5  = 12 Å S = –1.5  = 20 Å S = –1.8  = 12 Å S = –1.8  = 20 Å × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × × ×10 -6 Range of values

Figure courtesy of Gary Peterson, Breault Research Organization. Dust Contamination

The number of particles per square foot with diameters greater than s microns is given by: log(n) = [ (log(c)) 2 - (log(s)) 2 ] s = particle diameter (  m) c = cleanliness level n = number of particles per square-foot with diameters greater than s Courtesy of Gary Peterson, Breault Research Organization. MIL-STD 1246C

Buffington and Jackson Measurements are only available to within a degree of the specular direction. We know the linear relationship cannot go on indefinitely and retain a finite TIS. The roll-off will likely occur right in our angular domain, so knowledge of the position of the “knee” is critical to dust analysis.

Roll-off in the IR (10 microns) From Spyak and Wolfe, Scatter from particulate-contaminated mirrors, Part 3

The Mie Model for 0.01% Coverage (Level 230)

Dust results at 1 Micron

Dust accumulation

Accumulation with time

Rate of change ≈ 0.04% per hour! 40 Times faster at Apache Point

Kitt Peak Dust Experiment At what rate does dust accumulate in the McMath-Pierce tunnel? What is the distribution of particle sizes? What affect does an air knife have on dust accumulation rates?

A Large Compressor!

The Experiment

The Air Knife and Samples

Super Air Knife by Exair

24-Hour Accumulation 330  m 200  Magnification 10  m

The Need for Clean Air

Dust Scatter vs. Wavelength

Other Stray Light Sources

Relative Contributions Relative Contribution

Conclusions from the Reports “Scattering due to dust contamination of the primary mirror would appear to be the most serious stray-light concern for coronagraphic observations. The accumulation of dust on the primary quickly overwhelms the effects of surface microroughness from the polishing process.” M1 Microroughness and Dust Contamination:

Dust Dominates In situ washing is already part of the baseline plan. Operational procedures will have to be developed (as with any telescope) that establish criteria for “safe exposure” of the telescope to high winds in high-dust situations.